multi-material design in additive manufacturing—feasibility [email protected] s....
TRANSCRIPT
RESEARCH PAPER
Multi-material design in additive manufacturing—feasibilityvalidation
M. Leicher1 & S. Kamper1 & K. Treutler1 & V. Wesling1
Received: 21 October 2019 /Accepted: 8 March 2020# The Author(s) 2020
AbstractThe present investigations on generative manufacturing using metallic materials pursue the idea of transferring the microscopicstructural morphology of a dual-phase steel in modified form to the macroscopic level. The aim is to be able to join materials ofdifferent lattice modifications and to combine their positive properties. This applies in particular to the combination of hightensile strength and good formability. For this investigation, a specimen was created from a high-strength ferritic/martensitic(25%) and an austenitic (75%) material with a defined welding sequence. The specimen was deliberately manufactured aniso-tropically using welding layers in order to quantify its properties. Tensile tests were performed on specimens with different weldseam orientations to determine the direction-dependent properties. As can be proven by the results, the application of weldingprocesses with different materials results in an anisotropic behaviour in generative manufacturing. With regard to tensile strengthand elongation, there is an integral value of the mechanical-technological properties of both base materials. The existinganisotropy can be utilized with regard to the design by adapting the alignment of the weld layers to the load.
Keywords WAAM .Multi-material design . Additivemanufacturing
1 Introduction
The increasing importance of resource-efficient handling ofraw materials is driving development to design componentsmore efficiently. As a result, manufacturing processes are be-ing reorganized and the trend is that components are no longermilled from the “solid”, but generated using 3D printing
processes. Generative manufacturing processes, such as wirearc additive manufacturing (WAAM), can increase the degreeof material utilization [1–4]. The CMT arc welding technolo-gy is feasible for manufacturing bead structures [5–7].Furthermore, this process offers the possibility of a multi-material design. For this purpose, different materials are spe-cifically combined with each other in order to adjust localmechanical properties to the strains. For the targeted adjust-ment of the mechanical properties, the basic idea is to transferthe microscopic structure of the dual-phase steel or duplexsteel in modified form to the macroscopic level [7]. The com-bination of morphologies is intended to utilize advantages ofthe microstructures, thereby increasing the effectiveness andvariety of possible part designs. In addition, the arc-basedgenerative manufacturing offers the possibility to locally ad-just the material properties depending on the load, due to theprocess immanent conditions. This kind of multi-material de-sign is exclusive to additive manufacturing processes.
Furthermore, an anisotropic behaviour can be achieved byusing welding layers. The mechanical properties are specifi-cally enhanced by the alignment of the layers. The result ofthis targeted integration of materials and layer alignment is anexact adjustment of the mechanical properties to the existingstrains [8].
Recommended for publication by Commission I - AdditiveManufacturing, Surfacing, and Thermal Cutting
This article is part of the collection Additive Manufacturing – Processes,Simulation and Inspection
* M. [email protected]
1 Institut of Welding and Machining, TU Clausthal, Agricolastraße 2,Clausthal-Zellerfeld, Germany
https://doi.org/10.1007/s40194-020-00887-2
/ Published online: 8 May 2020
Welding in the World (2020) 64:1341–1347
2 Basics
The primary goal of the investigations is the validation of thewelding feasibility of a multi-material design. The microscop-ic structure of duplex and dual-phase steels, as shown sche-matically in Fig. 1, serves as a template for the layer structure.Duplex steels consist of equal parts of ferrite and austenite, asshown in Fig. 1a. They combine the advantages of both mi-crostructures. Ferritic steels have a high strength. Austeniticsteels, on the other hand, are characterized by high toughness.This combination produces very good mechanical and tech-nological properties [10].
Figure 1b shows the schematic representation of a dual-phase steel, which are characterized by a low carbon contentand consist of a ferritic matrix in which about 20% martensiteis embedded [11].
These two microstructures result in a fusion of the positiveproperties of the individual parts. This principle can, in a figurativesense, be applied to the macroscopic level. Above all, the combi-nation of the different advantages of the different materials shouldlead to an adaptation of the mechanical properties [12, 13].
A soft metal matrix reinforced with hard areas improves themechanical-technological properties with regard to strengthand toughness.
The specimen will be welded by stringer bead welding.The soft matrix is represented by the austenitic FeNi36 withthe standard designation ENNi36 (Table 1). The high-strengthpart is a filler metal Mn4Ni1,5CrMo with the standard desig-nation DIN EN ISO 16834-A G 69 [15] (Table 2).
Both the pure welding wires and the weld metal of theMn4Ni1,5CrMo are classified in the Schaeffler diagram in
a) b)
Fig. 1 Schematic structure. a Duplex steel. b Dual-phase steel [9]
Table 1 Chemical composition FeNi36 [14]
Main alloying additions in filler wire (wt.%)
C Si Mn P S Cr Mo Ni Co Fe
≤ 0.01 0.35 0.6 0.025 0.025 ≤ 0.5 ≤ 0.5 35–38 ≤ 1 Rest
Table 2 Chemical composition Mn4Ni1,5CrMo [16]
Main alloying additions in filler wire (wt.%)
C Si Mn Cr Ni Mo Ti
0.09 0.55 1.67 0.25 1.6 0.5 0.07
Fig. 2 Division of the layers (white: soft matrix, black: high-strengthdeposits)
Table 3 Process parameter
Filler metal FeNi36 Mn4Ni1,5CrMo
Stickout (mm) 15 15
Wire diameter (mm) 1.14 1.2
Wire feed (m/min) 4.5 5.0
Welding velocity (m/min) 0.3 0.3
Gas M21 M21
Current (I) 152 165
Voltage (V) 14.7 15.2
Process CMT CMT
Fig. 3 Welded specimen
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form of the nickel and chromium equivalent, with FeNi36being outside the evaluable range.
Formula for calculating the nickel equivalent [17]:
NiEqu ¼ %Niþ 30 �%Cþ 0:5 �%Mn ð1Þ
Formula for calculating the chrome equivalent [17]:
CrEqu ¼ %Cr þ%Moþ 1:5 �%Siþ 0:5 �%Nbþ 2 �%Ti ð2Þ
3 Experimental investigation
To produce the specimen, the layers of the FeNi36 werewelded first. The Mn4Ni1,5CrMo welding beads wereinserted into the gaps between the weld seams. This procedurecreates a soft matrix with high-strength components. Due tothe simple layer structure for welding, the soft matrix repre-sents in this first investigation a percentage of 75% of the test
specimen with an insular distribution of the high-strengthsteel. This can be seen schematically in Fig. 2.
3.1 Welding parameters
Due to the different welding filler materials, different weldingparameters are required to produce the specimen with alter-nating line beads. The parameters listed in Table 3 have showna particularly good surface quality between the individualbead layers.
To produce the specimen, three stringer beads of FeNi36were al ternately welded to one str inger bead ofMn4Ni1,5CrMo. As seen in Fig. 2, the white parts are theFeNi36 and were welded first from left to right and let gapson the black parts. These parts were filled after a change of thewelding wire with stringer beads of Mn4Ni1,5CrMo. In thislayer structure, test piece was built up a height of ten layers.The welded specimen is shown in Fig. 3.
3.2 Sample preparation
To investigate the mechanical-technological properties and apossible anisotropy of the multi-material design, tensile sam-ples were taken. These were manufactured in different orien-tations to the weld layers and then subjected to destructivetensile tests.
The alignments are transverse upright to the welding direc-tion, transverse flat to the welding direction and longitudinalupright to the welding direction. These are shown schemati-cally in Fig. 4. The different alignments are used to investigatewhether anisotropic behaviour occurs and whether multi-material construction is feasible.
a)
b)
c)
Fig. 4 Layer orientation of the tensile specimens (black representsMn4Ni1,5CrMo). a Transverse upright. b Transverse flat. cLongitudinal upright
Fig. 5 Schaeffler diagram withclassification of theMn4Ni1,5CrMo position [17]
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In addition, a microsection and a specimen for hardnessmeasurement were created from the welded specimen.T h e t r a n s i t i o n a r e a s b e t w e e n F eN i 3 6 a n dMn4Ni1,5CrMo were examined metallographically. Thejoining of the two materials and the absence of cracks,pores and inclus ions were careful ly examined.Furthermore, the microstructures are analysed.
4 Results
4.1 Structural morphology
The morphology of the microstructures is examined and theirsusceptibility to hot and cold cracking is classified with refer-ence to the Schaeffler diagram. Only the weld metal of the
Fig. 6 Layer 1 (left) and layer 2(right) of Mn4Ni1,5CrMo weldmetal
Fig. 7 Layer 3 (left) and layer 4(right) of Mn4Ni1,5CrMo weldmetal
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Mn4Ni1,5CrMo is classified by the diagram, since the FeNi36is outside the nickel equivalent and therefore cannot be deter-mined. The weld metal of the Mn4Ni1,5CrMo is calculatedwith a process-related mixing of 20%. According to formula(1), this results in a nickel equivalent NiEqu ,weld
metal,Mn4Ni1,5CrMo = 12.331%, and according to formula (2), achromium equivalent of CrEqu,weld metal,Mn4Ni1,5CrMo =1.676%. In comparison, the pure filler metal equals toNiÄqu,Mn4Ni1,5CrMo = 5.135% and CrÄqu,Mn4Ni1,5CrMo =1.715%. If these values are classified in the Schaeffler dia-gram, the points shown in Fig. 5 result. The pure filler metalMn4Ni1,5CrMo is in the purely martensitic range and is sus-ceptible to cold cracking. Through the combination withFeNi36, the Mn4Ni1,5CrMo weld metal is shifted in the di-rection of the martensitic-austenitic range. This shift increasesthe susceptibility to hot cracking because of 20% mixing withFeNi36, while the susceptibility to cold cracking below400 °C remains unchanged.
Figure 6 shows that layer 1 has a martensitic structure. Theneedle-like structure of the martensitic is homogeneously dis-tributed over the entire section. Compared with the otherlayers, it is finer-grained, since no further welding processtakes place over these layers and thus no additional heat influ-ence on the microstructure occurs. The black spots in themicrostructure are the smallest carbides formed from the melt.
Layer 2 also has a martensitic structure with austenitic andferritic islands, depends on the mixing in the stinger bead. Sincethis was reheated due to the layer welded above it, part of thecarbon, dissolved in the body-centred tetragonal martensite
lattice, could diffuse and thus fold down into austenite and abody-centred cubic ferritic lattice.
Figure 7 shows that layer 3 has larger areas of ferrite andaustenite than layer 1 or 2. This is due to the heat input by thewelding process. It is noticeable that the ferrite is acicular, as ithas no time to grow due to the cooling speed. This also applies tolayer 4. In contrast to layer 3, this microstructure is finer andmore homogeneous. Due to the influence of temperature, ce-mentite is precipitated at the ferrite grain boundaries by diffusionprocesses. In addition, no carbides are visible in the lower layers.
The microstructure of the cross section was examined to de-termine the microstructure morphologies of the individual phases.The high-strength weld seams made of Mn4Ni1,5CrMo have an
Fig. 8 Layer 3 (top) and layer 4 (down) of Mn4Ni1,5CrMo weld metal
Fig. 9 Hardness distribution
Table 4 Mechanical properties of base material according to data sheet[14, 16]
Material Rp0.2 (N/mm2) Rm (N/mm2) A (%)
FeNi36 310 490 30
Mn4Ni1,5CrMo 720 790 17
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austenitic-ferritic/martensitic structure. The soft matrix of FeNi36,on the other hand, consists entirely of austenite.
Figure 8 shows the transition of the high-strength beadsto the soft matrix. This shows two microstructures, whichare separate from each other, as it corresponds to the idea.Furthermore, neither pores, cracks nor inclusions are vis-ible. This shows that ferritic/martensitic and austeniticmicrostructures can exist side by side without a clear tran-sition zone. The mixing between the two layers is margin-al due to the low heat input. In order to avoid undesiredeffects such as brittle phase formation in the mixing zone,the two types of microstructure must be clearly separatedfrom each other.
4.2 Material characteristics
The hardness distribution diagram in Fig. 9 shows the homo-geneously distributed hardness of the FeNi36.The high-strength inclusions are clearly visible in the hardness measure-ment. This macrostructure resembles the microscopic appear-ance of a dual-phase steel and represents the basic idea of hardphases in a soft matrix.
To evaluate the mechanical properties, the tensile strengthof the welded test piece is determined. A possible anisotropycan be detected by comparing the yield strength and ductility.
Themechanical properties of the basematerials are listed inTable 4 for comparison.
Based on the mechanical properties of the base materials,the objective is to achieve an improvement in yield strengthand tensile strength compared with FeNi36 and to approxi-mate the yie ld s t rength and tens i le s t rength ofMn4Ni1,5CrMo, while increasing ductility. However,welding defects cannot be excluded. These reduce the elonga-tion at break under certain circumstances. The reason for thismay be the selected layer structure, which will be modifiedand optimized in further investigations.
The mechanical properties of the welding specimens inTable 5 show that the yield strength as well as the tensilestrength of the specimens is higher compared with FeNi36.The alignment with the respective tensile strengths and yieldstrengths is illustrated in Fig. 10. A supporting effect isachieved in the soft matrix with the high-strength incorpora-tion. If the yield strengths are calculated according to themixing rule using the phase fractions of FeNi36 andMn4Ni1,5CrMo from base metal values, this is calculated asRp0.2 = 412 N/mm2. This value is below the experimentallydetermined values of the multi-material design. The phasefraction of theMn4Ni1,5CrMo is higher, especially in the caseof the sample with its longitudinal edge, due to the alignment.This is where the yield strength reaches the maximum value.The calculated determination of the tensile strength also re-sults in a lower value than the experimental determination.The tensile strength calculated according to the mixing ruleis Rm = 565 N/mm2 from base metal values. The behaviour ofthe tensile strength is analogous to that of the yield strengthand withstands the highest loads when stress is parallel to thewelding direction.
If the tensile strength and ductility values are compared onthe basis of the different layer orientation, it can be seen that ananisotropic behaviour in the workpiece can be achieved byusing GMAW layers. In fact of welding defects, the ductilityof the test specimen shows a loss.
The tensile strength and ductility show the anisotropicproperties of the specimen based on the welding direction.
Table 5 Mechanical properties welding specimen (average of 5samples each)
Sample Rp0.2 (N/mm2) Rm (N/mm2) A (%)
Transverse upright 439.27 591.20 8.28
Transverse flat 432.73 568.60 7.34
Longitudinal upright 459.44 656.83 14.93
Fig. 10 Orientation of the tensilespecimens taken
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This can therefore be used in the wire arc additive manufactur-ing of load-adjusted workpieces.
5 Conclusion
The presented investigations show that a multi-materialcomponent design is achievable in arc-based generativemanufacturing. Furthermore, it was shown that throughthe multi-material design, the material properties can beadapted locally to the load, since the two materials can beprocessed with each other without a clear transition zone.The tensile tests have shown that the soft matrix is sup-ported by the high-strength inclusions and thus has ahigher yield strength and tensile strength. Welding defectsof the test specimen effect in a loss of ductility. In addi-tion, a greater increase in yield strength and tensilestrength can be achieved through the use of the multi-material design than would be expected through themathematical determination. After comparing the me-chanical properties for the different orientations, an an-isotropy of the material can be determined by usingGMAW processes. This is of great advantage with regardto resource-efficient, load-oriented design adapted to localproperties.
6 Outlook
The main focus is on investigating the ductility of the sam-ples. This should be improved by a different layer arrange-ment. As further investigations, the influences on the fa-tigue strength are to be examined more closely. The aim isto examine the crack growth in a targeted manner.Furthermore, follow-up tests with different metal combina-tions will follow.
Acknowledgements The authors thank the company Fliess for providingthe welding filler metals.
Funding Information Open Access funding provided by Projekt DEAL.
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